This invention relates to an oversampling clock recovery method.
In recent years, a high-speed protocol has been proposed in data transmission. For this purpose, high speed is requested in a clock recovery circuit and a phase locked loop (PLL). The clock recovery circuit carries out extraction of a clock signal from data transmitted in a high-speed transmission. The phase locked loop (PLL) frequency synchronizes a clock signal used in a circuit with a transmitted clock signal.
Various analog type clock recovery circuits has been proposed. One of them carries out a phase-comparison using a one-phase clock signal by making one leading edge of the clock signal correspond to one bit of data. Inasmuch as it is necessary in this circuit to make a data rate equal to a clock frequency, the clock frequency must be made a high-frequency wave of a level of gigahertz (GHz) when the data rate becomes the level of giga bit per second (Gbps). It is difficult to meet the request of the high-speed in the clock recovery circuit and the phase locked loop (PLL). For example, it is not easy to make an oscillation frequency of a voltage controlled oscillator (VCO) included in the phase locked loop (PLL) the high frequency of the level of GHz.
In order to meet such a request, an oversampling type clock recovery method and circuit have been proposed. The oversampling type clock recovery method and circuit sample transmitted data at a plurality of clock signals (multi-phase clock signals) having different phases. In the oversampling type clock recovery, phase comparison is carried out by making plural leading edges of the multi-phase clock signals correspond to one bit of data. According to the oversampling type clock recovery circuit, inasmuch as it is possible to use the clock signals having a frequency lower than the data rate, it is possible to meet the request of the high-speed.
The clock recovery carrying out phase comparison by making n leading edges of the multi-phase clock signals correspond to one bit of data is called an n-times oversampling. An 8-times oversampling is disclosed in Japanese Unexamined Patent Publication Tokkai No. Hei 9-233061 or JP-A 9-233061. A 2-times oversampling is disclosed in U.S. Pat. No. 5,633,899 issued to Alan Fiedler et al.
However, conventional oversampling methods are disadvantageous in that it is difficult to cope with a further high-speed of data transmission, in the manner which will later be described in conjunction with FIGS. 1A and 1B.